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Re: Seismic loads on retaining walls

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When subject to geologic forces from plate tectonics, the crust initially
strains (bends and shears) elastically.  
For pure axial loading, Hoke's Law gives the stress that accompanies this
strain.   As rock is stressed it stores 
strain energy.   When the stress exceeds the ultimate strength of the
rocks, they break and rapidly move into new position.    In the process
of rupturing, the strain energy is released and seismic waves are
generated.    This is the fundamental description of the Elastic Rebound
Theory of EARTHQUAKE generarion that both structural and
geotechnical engineers are very familiar.  

However, there are 2 approaches to SEISMIC design both which are
"correct" in their ways: DYNAMIC analysis (the solution rests on
VIBRATION THEORY, FEA and other advanced techniques that require computer
software analysis.)
and the STATIC analysis ( lateral force is calculated as some fraction of
DL and the UBC & IBC codifies this
analysis). 

Althoudh different in approach the static analysis method is based on
ENGINEERING LOGIC JUDGEMENT and can be traced back to VIBRATION THEORY.  
Here I would agree with Mark's remark that both structural engineers and
geothechnical engineers  do not have a deep undertanding of  the
vibration theories on which the matematical models are used for analysis
and the soil mechanics during an EQ.

I believe AASHTO has guidelines for calculating active seismic pressure
on retaining walls.  Regarding load combinations, AASHTO/BDS load
combination can be followed.  Caltrans always requires seismic forces to
be included in the design of abutments and similar structures.  
Geotechnical engineers in their report include seismic active pressures
for Caltrans projects in general.  (as competent geotechnical firms
should always do)  For
ordinary building projects, I have never seen geotechnical engineers
mention the words 'active seismic pressure' anywhere in the report, even
for major retaining wall structures.  They however never forget to
recommend 33% increase in allowable bearing capacity for wind/seismic
load cases. Seems kind or uncomplete report... some call
it a "limited geotechnical" report.

Hope this helps.

Desi J. Kiss, PE
http://djkeng.tripod.com/


On Sun, 20 Oct 2002 00:37:31 -0400 Mark Gilligan
<MarkKGilligan(--nospam--at)compuserve.com> writes:
> A Geotechnical Engineer I know explained that he does not provide
> recommendations for seismic soil forces on retaining walls unless
> specifically requested.  The rational is that when you look at the 
> seismic
> loads in thecontext of the factor of safety of the system of both 
> the
> regular soil loads and the wall its self there is little likelyhood 
> of
> failure.
> 
> I have had others explain to me that when you look at the real 
> forces on
> the wall during an earthquake they are much smaller than assumed.  
> The
> basic issue seems to be that when the ground is moving towards the 
> wall,
> the wall is moving in the same direction since both the soil and the 
> wall
> are forced to move in the same direction.
> 
> When combining soil seismic loads acting on the walls with seismic 
> loads
> from the superstructure, people seem to ignore a very basic fact.  
> The
> seismic loads on the walls from the soil are estimates of real loads 
> while
> the siesmic loads from the superstructure are fiticious loads 
> generated by
> dividing the elastic loads by an R factor.  When the superstructure 
> has
> considerable overstrength capacity the real siesmic forces the 
> structure
> will see are significantly greater than estimated.  Thus without 
> some
> adjustments the practice of adding soil induced seismic forces to 
> those
> from the superstructure is like adding apples and oranges.
> 
> It would not supprise me if this whole issue of siesmic loads from 
> the soil
> has not been well thought out and that we are spending a lot of mis
> directed energy on this issue.
> 
> I think that part of the problem is due to the fact that most 
> geotechnical
> engineers do not understand the assumptions made by structural 
> engineers. 
> In addition most structural engineers do not have a deep 
> understanding of
> how the soil  really acts.
> 
> Mark Gilligan
> 
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